Experimental investigation of thermal cycling effect on physical and mechanical properties of bedrocks in geothermal fields

Accepted Manuscript Experimental investigation of thermal cycling effect on physical and mechanical properties of bedrocks in geothermal fields Guan Rong, Jun Peng, Ming Cai, Mengdi Yao, Chuangbing Zhou, Song Sha PII: DOI: Reference:

S1359-4311(17)36153-7 https://doi.org/10.1016/j.applthermaleng.2018.05.126 ATE 12265

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

23 September 2017 29 March 2018 28 May 2018

Please cite this article as: G. Rong, J. Peng, M. Cai, M. Yao, C. Zhou, S. Sha, Experimental investigation of thermal cycling effect on physical and mechanical properties of bedrocks in geothermal fields, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.05.126

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Experimental investigation of thermal cycling effect on physical and mechanical properties of bedrocks in geothermal fields

Guan Rong1, 2, Jun Peng1, 2 *, Ming Cai3, Mengdi Yao1, 2, Chuangbing Zhou1, 2, 4 *, Song Sha1, 2 1. State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China 2. Key Laboratory of Rock Mechanics in Hydraulic Structural Engineering (Ministry of Education), Wuhan University, Wuhan 430072, China 3. MIRARCO, Laurentian University, Sudbury, Ont., Canada P3E 2C6 4. School of Civil Engineering and Architecture, Nanchang University, Nanchang, Jiangxi 330031, China

* Corresponding authors: Chuangbing Zhou [email protected]

Jun Peng [email protected]

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Abstract: High temperature associated with geothermal fields affects the performance of bedrocks. Evaluation of physical and mechanical behavior of rocks in the process of thermal cycling at high temperature is one of the main issue in this application, which is also the main topic of the present study. In this study, microscopic observation and uniaxial compression tests with acoustic emission (AE) monitoring were conducted on two bedrocks (i.e., marble and granite) after treatment with different thermal cycles at high temperature. It is found that the P-wave velocity decreases as the number of thermal cycle increases. The characteristic stress levels and Young’s modulus decrease with the increase of the number of thermal cycle in the treatment. The peak strain and the maximum volumetric strain show an increasing trend as the number of thermal cycle increases. After failure, more fragments are observed in specimens treated with more thermal cycles and the integrity is also found to be lower than specimens treated with less thermal cycles. The AE technique is able to capture the failure process and the associated micro-cracking behavior during loading. The degradation of macro-properties of the rocks is to a large extent attributed to the generation of grain boundary and intra-grain micro-cracks inside the rock specimens due to the applied thermal stress. Overall, the thermal cycling weakens the mechanical properties of rocks; however, the weakening effect will become not pronounced with the increase of the number of thermal cycle in the treatment if a high temperature is applied as in this study (i.e., 600 oC). Key works: Thermal cycling; P-wave velocity; Acoustic emission (AE) monitoring; Microscopic observation; Uniaxial compression test; Physical and mechanical properties

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1 Introduction Growing energy demand is one of the most important challenges facing the world today. As a renewable energy resource, geothermal energy has attracted much attention due to its potential in industrial and residential applications. Thermal energy storage (TES) is necessary to exploit and use the geothermal energy efficiently [1-5]. The temperature associated with TES system generally ranges from 150 oC to 650 oC [6]. To ensure high performance of TES system under a high temperature condition, bedrocks must meet high quality standards of many aspects. One of the issues associated with TES is the evaluation of physical and mechanical properties of bedrocks under high temperature conditions [7]. A good understanding of the thermal effect on the physical and mechanical properties of rocks is importance for cost-effective engineering design and long-term stability maintenance of TES systems. Laboratory tests have been widely used to characterize the thermal effect on physical and mechanical properties of rocks. Tests have been conducted under two types of high temperature conditions, i.e., syn-temperature and post-temperature conditions. In the first case, a rock test is conducted under real time high temperature condition [8-13]. However, due to inconvenience of this type of test and the strict requirements for test machines, it is much easier to study the physical and mechanical properties of rocks under a post-temperature condition. In this case, a rock specimen is first subjected to heating-cooling cycles, which can produce thermal damage to the specimen, and rock property test is then conducted under room temperature. In recent years, the influence of thermal damage on the physical and mechanical behavior of rocks under a post-temperature condition has been intensively investigated in laboratory testing. The results from these studies reveal that thermal treatment has a large influence on the physical and 3

mechanical properties of rocks. Macro-properties including hardness [14], P-wave velocity [15-16], Young’s modulus [17-18], compressive strength [19-21], tensile strength [22-24], and fracture roughness [25-27] generally show a decreasing trend as the applied temperature increases. However, most of these existing investigations focused on the physical and mechanical properties of rocks after exposing specimens to different temperatures. The behavior of rocks after treatment with different thermal cycles at high temperature has not been comprehensively studied yet. It is well known that the bedrock in a TES system generally experiences different heating/cooling cycles. The associated temperature can reach as high as 650 oC [6]. Therefore, how the thermal cycling affects the physical and mechanical behavior of bedrocks needs to be well investigated. The main aim of this paper is to investigate the effect of thermal cycling at high temperature on the physical and mechanical behavior of bedrocks used for geothermal energy storage. Various innovative microscopic monitoring and observation techniques such as acoustic emission (AE) [28], scanning electron microscope (SEM) [29], and X-ray computerized tomography (CT) [30] have been commonly used to elucidate the change in the macro-properties of rock in response to the thermal treatment and mechanical loading. It is found from previous studies that the degradation in the macro-properties with increasing treatment temperature is mainly associated with the generation of micro-cracks from thermal stress loading, which is usually referred to as thermal-induced micro-crack damage. In the present study, the thermal cycling effect on the physical and mechanical properties of two rocks (i.e., marble and granite) is experimentally investigated. Uniaxial compression tests with AE monitoring were conducted on the two rocks after treatment with different thermal cycles at high temperature. The influences of thermal cycling on the physical and mechanical properties such as

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P-wave velocity, stress–strain curves, rock strength, Young’s modulus, and failure mode were then examined and discussed. Microscopic observations on thin sections were also conducted to elucidate the thermal micro-crack damage induced from treatment with different thermal cycles.

2 Experimental design 2.1 Description of rock The studied rocks are dolomitic marble and granite. Two marble blocks with a rough dimension of 100 × 30 × 15 cm3 and two granite blocks with a rough dimension of 30 × 30 × 15 cm3 were collected from mines located in Pingnan City and Jinjiang City, respectively. Both cities are in Fujian Province, China. The rock samples were then shipped to Wuhan University, China for specimen preparation and testing. As shown in Fig. 1(a), the dolomitic marble, which is an anchimonomineralic metamorphic and carbonate rock, is white. The rock is relatively isotropic in texture and composition. It is fine-grained and the grain size is about 0.2 to 0.5 mm. A total of 28 marble specimens are prepared for mechanical and physical tests. The results from the X-ray diffraction analysis reveal that the rock is composed of more than 96.0% of dolomite, with a small amount of calcite and albite. Fig. 1(b) presents the grey granite specimens. The granite is an acidic igneous rock. It is fine-grained and the grain size is about 0.5 to 1.0 mm. A total of 32 granite specimens are prepared for laboratory testing. The results from the X-ray diffraction analysis reveal that the granite is composed of 31% albite, 25% quartz, 23% k-feldspar, 16% illite, and a small amount of chlorite and calcite.

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2.8837

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Fig. 1. Photographs of the rock specimens and X-ray diffraction patterns for (a) dolomitic marble and (b) granite. (color figure online) In the present study, cylindrical specimens were drilled out of the large rock blocks along the short dimension using a diamond coring bit with an inner diameter of 50 mm. The rock specimens were then cut to 100 mm long pieces, resulting in a height-to-diameter ratio of 2. The ends of all specimens were finely polished to meet the specifications recommended by ISRM [31]. Physical properties of the rock specimens were measured prior to thermal treatment. The P-wave velocity of the specimens was measured through an ultrasonic pulse transmission technique and the average P-wave velocities for dolomitic marble and granite are 4.758 and 4.235 km/s, respectively. In addition, density and porosity of the rocks were determined using the methods recommended by ISRM [31]. Table 1 presents the detailed physical properties of the two rocks tested in this study. Table 1 Summary of physical properties of the two rocks before thermal treatment (standard

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deviation in parentheses). Rock type

Grain size (mm)

Bulk density (g/cm3)

Saturation density (g/cm3)

Total porosity (%)

Effective porosity (%)

P-wave velocity (km/s)

Marble Granite

0.2-0.5 0.5-1.0

2.713 (±0.001) 2.717 (±0.002)

2.726 (±0.004) 2.727 (±0.004)

1.8 (±0.15) 2.0 (±0.14)

1.3 (±0.07) 1.0 (±0.07)

4.758 (±0.024) 4.235 (±0.018)

2.2 Testing procedure In order to study the thermal cycling effect on the physical and mechanical properties of the rocks, the specimens were heated to a pre-determined temperature with different cycles (Fig. 2(a)). The heating device used in this study is a SX3-10-12 box-type resistance furnace, which is composed of a control box and a furnace. The maximum operating temperature is 1200 oC and the rated power is 10 kW. Heating a rock specimen to a pre-determined temperature and then cooling it down to the room temperature is regarded as one thermal cycle. In the process of a thermal cycle, the rock specimens were put into the furnace and heated to the pre-determined temperature at a heating rate of 10 oC/min. The pre-determined temperature, once reached, was kept constant for 4 hours in the furnace. The specimens were then taken out of the furnace and cooled down to the room temperature (25 oC) naturally. Based on the experience of a recent study [32], the pre-determined temperature used in this study was set to 600 oC to mimic a high temperature environment for deep bedrocks in a TES system. The numbers of thermal cycle investigated in the present study were 0 (i.e., no thermal treatment), 1, 2, 4, 6, 8, and 16. Before conducting the uniaxial compression test, P-wave velocities of the rock specimens after treatment with different numbers of thermal cycles were measured using an ultrasonic pulse transmission technique. As shown in Fig. 2(b), two transducers were positioned in the center of two ends of a specimen. White Vaseline was applied to eliminate the air between the specimen and

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transducers to ensure an effective energy transfer to the transducers. The transducers were carefully calibrated before testing and the wave frequency used in this study was 1 MHz. The specimens were air-dried before the ultrasonic test. Each specimen was repeatedly tested five times to ensure reliable test results. In addition to the P-wave velocity measurement, thin sections of the rock specimens after treatment with different numbers of thermal cycles were prepared and then observed under an optical microscopy. A cross-polarized light was used during observation of thin sections. To ensure reliability of the results, a total of nine thin sections at different positions were prepared in each specimen and examined for each thermal cycling condition (Fig. 2(b)). The microstructures inside the specimen were uniformly enlarged by a factor of 50 in this study. Uniaxial compression tests were performed using a hydraulic servo-controlled test system with a maximum load capacity of 3000 kN and a maximum confining pressure capacity of 100 MPa. The measurement ranges of the axial and lateral displacement meters, which were carefully calibrated before testing, were 8 and 4 mm, respectively. The nonlinearity of the two meters was less than 0.01% of the full-scale measurement range. In this study, the axial stress was applied using axial displacement-controlled loading at a rate of 0.075 mm/min. In all uniaxial compression tests, AE activities were recorded using AE detection model PCI-2 of the DISP series manufactured by American Physical Acoustic Corporation. Four AE piezo ceramic transducers with a resonance frequency range of 100 to 400 kHz were directly mounted onto the surface of a specimen. As illustrated in Fig. 2(c), two AE transducers (1# & 2#, 3# & 4#) were treated as a group and installed symmetrically in a plane at about 1/4 of the height of the specimen. An orthogonal layout of the two groups of transducers provided a good coverage of the

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specimen volume. To achieve a satisfactory acoustic coupling, a thin layer of white Vaseline was applied at the interface between the specimen surface and the AE transducers [33-35]. Two loops of rubber band with the width equal to the AE transducer diameter was used to fix the transducers. To check the coupling between the AE transducers and the specimen surface, pencil lead fracture tests were conducted before AE monitoring. Each AE transducer was calibrated to have a minimum amplitude of 95 dB. (a)

(b)

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2#

Rock specimen AE transducers 3#(4#) Thin sections

Induced thermal micro-cracks Heating

Transducer for P-wave velocity testing

Cooling

Thermal cycling treatment

Microscopic tests

Axial stress

Uniaxial compression test

Fig. 2. Illustration of experimental procedure used in this study. (a) Thermal cycling treatment; (b) P-wave velocity measurement and microscopic observation of thin sections; and (c) uniaxial compression test with AE monitoring.

3 Physical and mechanical properties 3.1 P-wave velocity The variations of P-wave velocity with the number of thermal cycle for the two rocks are presented in Fig. 3. Both rocks show similar trend. The results reveal that the thermal treatment has a large effect on the P-wave velocity of the rocks. The P-wave velocity reduces noticeably with just one cycle of thermal treatment. The decrease in the P-wave velocity gradually diminishes as the number of thermal cycle increases. In general, the P-wave velocity decreases by 73.2% and 72.6% 9

for marble and granite, respectively. The P-wave velocity is very sensitive to the presence of micro-defects such as micro-cracks and voids in the rock specimens. The decrease in the P-wave velocity is, to a large extent, associated with the development of micro-cracks induced from thermal loading. The trend shown in Fig. 3 indicates progressive development of micro-cracks inside the specimens as the number of thermal cycle increases. The observation can be validated from the results obtained from microscopic examination of thin sections (see Section 4). The results are also in a good agreement with those obtained from previous laboratory studies [15-16]. 6

Marble Granite

P-wave velocity (km/s)

5 4 3 2 1 0 0

2

4

6

8

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14

16

18

Number of thermal cycle

Fig. 3. Variation of P-wave velocity with the number of thermal cycle for the two rocks. 3.2 Stress–strain relation Fig. 4 shows the axial stress–axial strain and the axial stress–volumetric strain relations of the specimens after treatment with different thermal cycles. These relations were obtained from uniaxial compression tests. The results reveal that the thermal cycling influences the stress–strain relations. With the increase in the number of thermal cycle, the slope in the initial deformation stage of the axial stress–axial strain curves decreases. As pointed out by Peng et al. [36], the slope in the 10

initial deformation stage can partly represent the thermal-treatment-induced micro-crack damage in the rock specimen. As the number of thermal cycle increases, more micro-cracks are developed inside the specimen, resulting in a larger irreversible deformation which is mainly associated with the closure of micro-cracks during loading. (a) 70

(b) 70 0 cycle 1 cycle 2 cycles 4 cycles 8 cycles 16 cycles

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Fig. 4. Recorded stress–strain relations of the two rocks with different numbers of thermal cycles. (a) Axial stress versus axial strain curves and (b) axial stress versus volumetric strain curves for the marble specimens; (c) axial stress versus axial strain curves and (d) axial stress versus volumetric strain curves for the granite specimens. (color figure online) The post-peak stress–strain curve is also affected by the thermal cycling. When the number of thermal cycle is small, the post-peak stress–strain curves of the marble specimens show a brittle behavior (i.e., the strength drops rapidly when the peak stress is reached). As the number of thermal cycle increases, the ductility of the specimen is enhanced by thermal damage. Because the granite 11

tested in this study is a hard rock and the failure process is generally violent, a test was terminated once the peak strength was reached to avoid damage of the test machine. Therefore, the post-peak behavior for granite specimens is not discussed in this study. 3.3 Rock strength and deformation behavior The strength and deformation behavior of rocks is mainly associated with the closure, initiation, propagation, and coalescence of micro-cracks developed inside the rock and the recorded stress–strain relation divides the rock deformation into several stages from crack closure to crack coalescence [37-38]. As such, several characteristic stress thresholds have been identified which are normally referred to as crack closure stress, crack initiation stress, crack damage stress, and peak strength. The crack closure stress is generally associated with the closure of existing micro-cracks in the rock specimen, which represents the degree of initial micro-crack damage to some extent. This stress threshold can be objectively determined using the axial strain response (ASR) method [39]. The crack initiation stress corresponds the point at which the deformation of rock changes from elastic to stable cracking. Crack initiation generally starts at a stress level of approximately 0.3 to 0.6 times of the peak strength. The lateral strain response (LSR) method [40] and the cumulative AE hit (CAEH) method [41] provide objective methods for the determination of the crack initiation stress. The crack damage stress denotes the onset of unstable crack growth which generally occurs at a stress level of approximately 0.7 to 0.8 times of the peak strength. The reversal point in the axial stress versus volumetric strain curve is usually used to determine the crack damage stress [38]. The peak strength represents the maximum loading capacity that a rock can sustain. In the following discussion, the relation between these characteristic stress thresholds and the number of thermal cycle is examined

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and analyzed. The variations of these characteristic stress thresholds with the number of thermal cycle for the marble and the granite are presented in Fig. 5(a) and Fig. 5(b), respectively. Both rocks show similar trends. The peak strength, crack damage stress, and crack initiation stress generally show a decreasing trend with the increase in the number of thermal cycles. The decrease in these three stress thresholds gradually diminishes as the number of thermal cycle increases. These stress thresholds generally decrease by 71.5% and 51.5% for marble and granite, respectively. The results reveal that thermal cyclic loading induces micro-crack damage to the rock specimen and thus reduces the characteristic stress thresholds. On the other hand, the crack closure stress seems less affected by the thermal cycling, particularly for the granite. This indicates that though more micro-cracks will be generated with more numbers of thermal cycling treatment, these thermally-induced micro-cracks are easily closed with similar load during uniaxial compression tests.

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(a)

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Peak strength Crack damage stress Crack initiation stress Crack closure stress

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Number of thermal cycle Peak strength Crack damage stress Crack initiation stress Crack closure stress

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Fig. 5. Variations of characteristic stress thresholds with the number of thermal cycle for (a) marble and (b) granite. Young’s moduli of the rock specimens after treatment with different numbers of thermal cycles are also examined. In this study, the Young’s modulus refers to the tangent modulus determined from the linear portion of the stress–strain curve between 40% and 60% of the peak stress. Fig. 6 presents the variations of Young’s moduli with the number of thermal cycle for the two rocks, showing a gradual decreasing trend with the increase in the number of thermal cycle. The decrease in Young’s modulus gradually diminishes as the number of thermal cycle increases. In total, the Young’s 14

modulus decrease by 88.9% and 67.6% for marble and granite, respectively. 42

Marble Granite

Young's modulus (GPa)

36 30 24 18 12 6 0

0

2

4

6

8

10

12

14

16

18

Number of thermal cycle

Fig. 6. Variation of Young’s modulus with the number of thermal cycles for the two rocks. The peak strain is defined as the axial strain that corresponds to the peak strength. The parameter is commonly used in rock mechanics to evaluate deformation properties of rocks. The variation of peak strain with the number of thermal cycle is shown in Fig. 7. The peak strains of both rocks gradually increase as the number of thermal cycle increases. 15

-3

Peak axial strain (10 )

12

9

6

Marble Granite

3

0

0

2

4

6

8

10

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14

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Fig. 7. Variation of peak axial strain with the number of thermal cycle for the two rocks.

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The maximum (positive) volumetric strain denotes the maximum amount of contraction of a rock that previously experienced during loading, which reflects the compressibility of the rock to some extent. Fig. 8 presents the variations of maximum volumetric strains with the number of thermal cycle for the two rocks. The results reveal that the maximum volumetric strain generally shows an increasing trend with the increase in the number of thermal cycle. Because more micro-cracks are generated in a specimen with more thermal cycles, the compressibility of the specimen will be more prominent, resulting in a larger volumetric compression deformation.

-3

Maximum volumetric strain (10 )

12 10 8 6 4

Marble Granite

2 0

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Fig. 8. Variation of maximum volumetric strain with the number of thermal cycle for the two rocks. 3.4 Acoustic emission characteristics AE is defined by the transient elastic wave generated by the rapid release of energy from a source within a material [42-43]. AE activities in a rock generally initiate from micro-cracking which is associated with dislocations, grain boundary movement, or initiation, propagation, and coalescence of micro-cracks through and between mineral grains [44]. AE monitoring has been widely used in laboratory tests to study the micro-cracking behavior of rocks during loading [45-46]. In the following discussion, the AE characteristics of the specimens for both rocks after thermal 16

treatment with different numbers of cycles are examined and analyzed. Fig. 9 presents the variations of AE hits and accumulated AE counts with the axial strain for the marble specimens after treatment with different numbers of thermal cycles, along with the stress–strain relations. The results reveal that thermal cyclic loading has a large influence on the AE characteristics of the rocks and the evolutions of accumulated AE counts and AE hits correlate well with the stress–strain relations. In general, almost no AE signals can be recorded in the initial deformation stage for the specimens that received no thermal treatment. The AE activities initiate at a stress level about 0.4 times of the peak strength and then increase rapidly at about 0.8 times of the peak stress.

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Fig. 9. Evolutions of axial stress (black), AE hit rate (blue), and accumulated AE counts (red) with axial strain for the marble specimens after treatment with different thermal cycles. (a) 0 cycle, (b) 1 cycle, (c) 2 cycles, (d) 4 cycles, (e) 8 cycles, and (f) 16 cycles. (color figure online) As the number of thermal cycle increases, the AE activity becomes more drastic, which is evident from the increases in the amount of recorded AE hits and accumulated AE counts. In addition, with the increase in the number of thermal cycle, the AE signals in the initial deformation stage become more prominent. This is because more micro-cracks have been generated in the rock specimens with more number of thermal cycling treatment. These thermally-induced micro-cracks 18

result in more AE activities in the initial deformation stage. The stress–strain relations and the corresponding variations of AE hits and accumulated AE counts with the axial strain for the granite specimens with different numbers of thermal cycling treatment are presented in Fig. 10. The results reveal that the thermal cycling has a large influence on the AE characteristics of the granite specimens. The evolution of AE activities correlates well with the stress–strain curve. In the initial deformation stage, specimens with thermal cycling treatment have more AE activities than these without thermal cycling treatment. In the subsequent deformation, systematic AE activities appear at a stress level about 0.4 times of the peak strength and then multiply at about 0.8 times of the peak stress, which is similar to the results of the marble specimens.

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Fig. 10. Evolutions of axial stress (black), AE hit rate (blue), and accumulated AE counts (red) with axial strain for the granite specimens after treatment with different thermal cycles. (a) 0 cycle, (b) 1 cycle, (c) 2 cycles, (d) 4 cycles, (e) 8 cycles, and (f) 16 cycles. (color figure online) Compared with those of marble specimens, there are more AE activities in the granite specimens. In addition, as the number of thermal cycle increases, the AE activity in the granite specimens becomes less prominent, which is opposite to the results of the marble specimens. This is mainly due to the fact that the studied granite is more brittle and harder than the dolomitic marble. The elastic energy stored in granite is higher than that of marble during the uniaxial compressive

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loading. Hence, the detected AE activities are more prominent in granite than in marble, which is evident from the absolute values of AE hits and accumulated AE counts shown in Fig. 9 and Fig. 10. However, as the number of thermal cycle in the treatment increases, more micro-cracks are developed and the ductility is gradually enhanced for the granite specimens. Less elastic energy will be stored in granite, resulting in a less prominent AE activity with the increase in the number of thermal cycling treatment. The real-time AE source location method is widely used in rock mechanics to determine the micro-cracking evolutions of a rock specimen during loading [47]. As an example, Fig. 11 presents the evolutions of the accumulated AE events in a marble specimen at different deformation stages after thermal treatment with 6 cycles. The AE counts increase gradually and multiply as the specimen is loaded to failure. The AE event cloud finally forms a macroscopic failure plane in the specimen, which is oriented approximately in the vertical direction (Fig. 11(b)). The interpreted failure plane by the AE count cloud is comparable with that observed directly in uniaxial compression test (Fig. 11(c)).

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Fig. 11. Locations of accumulated AE events in the marble specimen after 6 thermal cycles at different deformation stages. (a) Complete stress−strain curve associated with evolution of AE counts; (b) spatial distribution of accumulated AE counts in the specimen; and (c) experimentally observed splitting fractures approximately parallel to the loading direction. (color figure online) 3.5 Failure mode The failure modes of the marble specimens after treatment with different numbers of thermal cycles are presented in Fig. 12. The specimens fail primarily in a splitting manner but the integrity of the rock specimen after failure is different. The integrity of the specimens experienced more thermal cycles is lower than specimens with less thermal cycle treatment. This is because more thermally-induced micro-cracks had been developed in specimens that received more thermal cycles. As the number of thermal cycling treatment increases, the elastic energy stored in the specimen and testing machine gradually decreases. Hence, more fragments are observed after failure in the specimens with more numbers of thermal cycling treatment.

Fig. 12. Failure modes of the marble specimens after treatment with different numbers of thermal cycles. (a) 0 cycle, (b) 1 cycle, (c) 2 cycles, (d) 4 cycles, (e) 8 cycles, and (f) 16 cycles. (color figure online) The studied granite is more brittle and harder than the dolomitic marble. The elastic energy in the granite is generally higher than that in the marble. Hence, the failure of the granite specimens is 23

more violent. As shown in Fig. 13, the granite specimens generally disintegrate in several pieces. The integrity of the granite specimens after failure is lower than that of the marble specimens. Similar to the results of the marble specimens, more fragments are observed after failure when the granite specimens experienced more numbers of thermal cycling treatment.

Fig. 13. Failure modes of the granite specimens after treatment with different numbers of thermal cycles. (a) 0 cycle, (b) 4 cycles, and (c) 8 cycles. (color figure online)

4 Microscopic observation Thermal cycling treatment can cause a change in the microstructure of the rocks, which is mainly associated with initiation and propagation of micro-cracks along the mineral boundaries or inside the mineral grains. An optical microscopy study on thin sections was conducted to directly observe micro-cracks in the specimens subjected to thermal treatment with various cycles before mechanical tests. It should be noted that the examined thin sections were not extracted from the same specimen, neither from the same position. The results of the optical microscopy observation can only be used as a qualitative analysis for the identification of micro-crack variation in the specimens with different numbers of thermal cycling treatment. Fig. 14 presents the observation results of the thin sections for the marble specimens. The 24

results reveal that the mineral grains and the associated micro-cracking are affected by thermal cycling treatment. More thermal micro-cracks are induced as the number of thermal cycle increases. The apertures of the grain boundaries increase (i.e., grain boundary micro-cracks) with the increase of the thermal cycle in the treatment. In addition, several micro-cracks are observed to initiate and propagate inside the mineral grains, which are associated with intra-grain micro-cracking, when the specimen experiences several numbers of thermal cycling treatment.

Fig. 14. Thin sections of the marble specimens after treatment with different numbers of thermal cycles (1 – grain boundary cracks (red), 2 – intra-grain cracks (blue)). (a) 0 cycle, (b) 1 cycle, (c) 8 cycles, and (d) 16 cycles. (color figure online) When there is no thermal damage (i.e., no thermal cycling treatment), very few micro-cracks are observed inside the specimen and the grains are well cemented with each other. When a rock

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specimen experienced a small number of thermal cycling treatment, heat expansion causes separation of some grains along the grain boundaries. A large amount of micro-cracks can be mostly observed in the grain boundaries. When the applied number of thermal cycle is large, more intra-grain micro-cracks can be observed. The thin sections for the granite specimens with different numbers of thermal cycling treatment are presented in Fig. 15. Thermal cycling treatment influences the mineral grains and the associated micro-cracking behavior. More thermal micro-cracks (both grain boundary micro-cracks and intra-grain micro-cracks) are observed in the rock specimens as the number of thermal cycle increases. Compared with the results shown in Fig. 14, the grain structure in the granite specimens is not regular and it is difficult to see clearly the development of intra-grain micro-cracks relative to the grain boundary micro-cracks.

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Fig. 15. Thin sections of the granite specimens after treatment with different numbers of thermal cycles (1 – grain boundary cracks (red), 2 – intra-grain cracks (blue), Qtz – Quartz, KFs – K-feldspar, Ill – Illite, Ab – Albite, Bt – Biotite (minerals in pink)). (a) 0 cycle, (b) 1 cycle, (c) 8 cycles, and (d) 16 cycles. (color figure online) Overall, thermal cycling treatment induces micro-cracks inside the rock specimen. As the number of thermal cycling treatment increases, more micro-cracks, especially the intra-grain micro-cracks, can be observed. Hence, the intra-grain micro-cracking is an effective index to evaluate the thermally-induced micro-crack damage. Though the direct thin section observation in this study provides qualitative information for the micro-crack development, quantitative technologies, such as X-Ray diffraction analysis (XRD) and thermo-gravimetric analysis (TGA) can provide more useful details for the mineral decomposition and transformation in response to thermal treatment. These technologies are recommended to use in future studies of thermal effect on performance of bedrocks in geothermal fields.

5 Conclusions This study experimentally investigates the thermal cycling effect on the physical and mechanical properties of two rocks (i.e., dolomitic marble and granite). Microscopic observations and uniaxial compression tests with AE monitoring were conducted on rock specimens after different numbers of thermal cycling treatment at a temperature of 600 oC. The test results reveal that the thermal cycling has a large influence on the physical and mechanical properties (i.e., P-wave velocity, stress–strain curves, rock strength, Young’s modulus, and failure mode) of the rocks. The mechanical behavior of dolomitic marble and granite in response to the increase in the number of thermal cycle generally shows a similar trend. The P-wave velocity shows a decreasing 27

trend as the number of thermal cycling treatment increases. The characteristic stress thresholds and Young’s modulus also decrease with the increase in the number of thermal cycling treatment. The peak strain and the maximum volumetric strain show an increasing trend as the number of thermal cycle increases. After failure, more fragments are observed and the integrity of the specimens with more thermal cycling treatment is low when compared with the specimens with less thermal cycling treatment. In addition, the failure of granite specimens is more violent when compared with that of marble specimens. The recorded AE activities correlate well with the deformation behavior during loading and the failure plane interpreted by the AE count cloud is comparable with that observed directly in the uniaxial compression test. The results obtained from microscopic observation on thin sections reveal that the degradation of the macro-properties of the rocks is to a large extent attributed to the generation of grain boundary and intra-grain micro-cracks inside the rock specimen due to the applied thermal stress. The dominant mechanism is the generation of grain boundary micro-cracks. Overall, the experimental results in this study reveal that thermal cyclic loading can change the physical properties and weaken the mechanical properties of rock. If a high treatment temperature is applied as in the present study (i.e., 600 oC), the mechanical properties such as wave velocity, strength, and modulus decrease as the number of thermal cycling treatment increases. However, the decrease in these properties is prominent only in the first several thermal cycling treatments and gradually diminishes with the increase in the number of thermal cycling treatment. These mechanical properties can decrease by 50% to 90% in total for the two rocks. Hence, special attention should be paid to the long-term stability of the bedrocks in geothermal fields.

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Acknowledgements The authors would like to thank the three anonymous reviewers for their constructive comments and suggestions which greatly improved the quality of the manuscript. The research work presented in this paper is in part supported by the National Natural Science Foundation of China (Grant nos. 51579189, 51609178, and 41772305), the Fundamental Research Funds for the Central Universities (Grant nos. 2042016kf0171 and 2042016kf0042), the China Postdoctoral Science Foundation (Grant no. 2015M582273), and the Open-end Research Fund of the State Key Laboratory for Geomechanics and Deep Underground Engineering (Grant no. SKLGDUEK1709). The authors are grateful to these financial supports.

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Highlights:  Influence of thermal cycling on performance of rocks is experimentally studied.  Thermal cycling treatment significantly weakens mechanical properties of rocks.  Mechanical properties decrease by 50% to 90% in total for studied rocks.  Degradation of rock property is mainly associated with generated micro-cracks.  Acoustic emission and thin section observation capture micro-cracking behavior.

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